Ion implanters have been used for many years in the processing of semiconductor wafers. Typically, a beam of ions of a required species is produced and directed at a wafer or other semiconductor substrate, so that ions become implanted under the surface of the wafer. Implantation is typically used for producing regions in the semiconductor wafer of altered conductivity state, by implanting in the wafer ions of a required dopant.
A number of arrangements for generating a source of ions in an ion implanter are known. Hot cathode sources, such as the so-called Freeman or Bernas sources, use a directly heated filament to generate a source of thermionic electrons. The cathode is held at a high negative potential relative to an anti-cathode (usually formed from the walls of an arc chamber) and an arc current flows through an admitted gas supply to generate a plasma.
Alternatively, a microwave or rf source can be used. Here, a microwave or rf field excites free electrons which then ionise an admitted gas to again produce a source of ions for implanting.
In one common arrangement, known as a triode structure, a suppression or extraction electrode is used to extract the ions from the ion source where they are formed. The extraction electrode is arranged adjacent to an extraction aperture formed in a face plate mounted upon the arc chamber of the ion source. The potential difference between the arc chamber and the extraction electrode defines the energy of the resultant ion beam. The triode structure also includes a ground electrode prevent electrons from being swept away and thus allows ion beam neutrality to be preserved. The face plate, suppression or extraction electrode and ground electrode are henceforth termed an extraction assembly.
To permit acceleration of ions out of the ion source, the extraction electrode needs to be at a net negative potential with respect to the ion source itself. Thus, the ion source is typically electrically insulated from the extraction electrode by high voltage bushing formed from, for example, a ceramic based material A second, less common form of ion source assembly employs a tetrode structure. Here, instead of a dual purpose extraction/suppressor electrode such as is used in the triode structure described above, separate suppressor and extraction electrodes are employed. The suppressed electrode is electrically insulated from the suppressor electrode and is held at a net negative potential (for positively charged ions) with respect to it. Examples of tetrode structures are shown in U.S. Pat. No. 5,866,909 and WO99/23685.
In both the triode and tetrode structures, the ion source, isolated from the extraction assembly, is mounted coaxially within a first end of an elongate, usually cylindrical vacuum chamber. The other, second end of the vacuum chamber is mounted, often non-removably, around an inlet into a mass analyser.
The various parts of the ion source assembly (consisting of the ion source, extraction assembly, insulators and vacuum chamber) require frequent cleaning and servicing to prevent contamination of the resultant ion beam. For this reason, the ion source assembly must be dismantled.
Such a process is difficult and time consuming. The trend to larger ion implanters has in turn caused larger ion source assemblies to evolve, which tend to be relatively heavy. To dismantle such assemblies can require two persons or even lifting equipment. Furthermore, the particular shape of some components of typical ion source assemblies can in any event make them difficult to remove without damage. For example, the extraction electrode in the tetrode structure shown in WO99/23685 is mounted upon the base of a `cup` shaped electrode support of relatively small diameter. The elongate ion source then extends into the cup such that a front face of the ion source is generally parallel with, and adjacent to, the base of the cup (and the extraction electrode in particular). Then, even when the ion source is removed, the extraction electrode can only be accessed via the narrow diameter of the cup.